1. Field of the Invention
The present invention relates to a method and apparatus for producing a manganese-zinc ferrite single crystal, and more particularly to a method and apparatus for producing a manganese-zinc ferrite single crystal using a local liquid pool formation which involves melting of a crystal starting material in a crucible and lowering of the temperature of crucible to make the resultant melt maintain locally a liquid phase, that is, the melt form three phase regions of a liquid pool region, a semi-rigid region and a solid region.
2. Description of the Prior Art
Generally, a manganese-zinc ferrite single crystal is used as a core material for video heads and is formed in a crucible. However, such a manganese-zinc ferrite single crystal is likely to include an impurity, such as platinum, from the crucible during its formation in the crucible, resulting in a degradation in quality or an occurrence of defects.
For forming such a manganese-zinc ferrite single crystal, there has been conventionally used a floating zone method. However, this method enables only the formation of a single crystal having a diameter of below 10 mm. In producing a single crystal of larger diameter, for example, not less than 10 mm, a Bridgeman method has been mainly used.
In accordance with the Bridgeman method which is illustrated in FIG. 1, first, a seed crystal 23 and a starting material are charged into a platinum crucible 24. The interior of crucible 24 is maintained at a temperature making it possible to provide a constant temperature gradient using heaters 21 and 22 so that the charged starting material is melted. Thereafter, as the crucible 24 is moved downwardly at a proper rate, a single crystal is grown.
Where the bridgeman method is used, a solidification proceeds during the growth of single crystal, resulting in an increase in solidified portions. This increase in solidified portions causes a variation in shift rate of actual solid-liquid interface.
As a result, it is difficult to grow the single crystal under the constant condition throughout the whole length of an ingot. Upon shifting of the solid-liquid interface from the seed crystal to a cone-shaped portion, in particular, if the temperature gradient is improperly controlled at the cone-shaped portion, a phenomenon that the solid-liquid interface forms a shape concaving toward the liquid phase is increased, thereby causing the formed single crystal to be broken into polycrystal or to have defects.
Furthermore, an increased deviation in composition in the growth direction occurs due to an increased evaporation of ZnO generated at the surface of melt and a coring phenomenon caused by non-equilibrium solidification, resulting in a variation in magnetic characteristic along the length of ingot.
For decreasing the deviation in composition, therefore, there has been proposed a method of forming a single crystal using a continuous feeding-type Bridgeman technique wherein starting material tablets of a controlled composition are fed to an auxiliary crucible, to be melted before they are fed to a main crucible. In this method, however, an inclusion of platinum from the auxiliary crucible into a formed single crystal occurs, thereby adversely affecting the single crystal.
Since the melt is in close contact with the inner wall of crucible in the cases of using the conventional Bridgeman methods, stress is also generated which causes a dislocation in the formed single crystal. Such a dislocation undesirably results in a formation of subgrains upon crystal growing and cooling. In the conventional methods, moreover, the platinum crucible should be regenerated after every use thereof, resulting in an increase in manufacture cost.